Here's how to remove yourself from one Challenge to join a different one:

1. Click the top left of the app screen until you reach the "My Challenge" page. Please note the click box may be an arrow or it may be a box with an "X" in it as shown here:

2. On "My Challenge" page, click the BIG ICON to enter your current Challenge, as shown below:

3. Click the "Unsubscribe" button to remove yourself from the current Challenge, as shown below:

4. Enter the INVITE CODE for the next Challenge you wish to join, in the white box at bottom of the app:

Then press "Submit" to get into the game!  

If you don't know how to find your INVITE CODE, click here for more.